Single-package bridge-type magnetic field sensor

ABSTRACT

A magnetoresistive sensor bridge utilizing magnetic tunnel junctions is disclosed. The magnetoresistive sensor bridge is composed of one or more magnetic tunnel junction sensor chips to provide a half-bridge or full bridge sensor in a standard semiconductor package. The sensor chips may be arranged such that the pinned layers of the different chips are mutually antiparallel to each other in order to form a push-pull bridge structure. The sensor chips are then interconnected using wire bonding. The chips can be wire-bonded to various standard semiconductor leadframes and packaged in inexpensive standard semiconductor packages. The bridge design may be push-pull or referenced. In the referenced case, the on-chip reference resistors may be implemented without magnetic shielding.

PRIORITY CLAIM TO RELATED APPLICATIONS

This application is a continuation application and claims the benefit ofpriority of U.S. patent application Ser. No. 13/979,721, filed 15 Jul.2013, which is a national stage application under 35 U.S.C. §371 ofPCT/CN2011/085124, filed 31 Dec. 2011, and published as WO2012/097673 on26 Jul. 2012, which claims priority to China Application No.201110008762.2, filed 17 Jan. 2011, and to China Application No.201110141214.7, filed 27 May 2011, which applications and publicationare incorporated by reference as if reproduced herein and made a parthereof in their entirety, and the benefit of priority of each of whichis claimed herein.

FIELD OF THE INVENTION

The present invention relates to the field of magnetic field measurementusing a magnetic tunnel junction (MTJ, Magnetic Tunnel Junction) orgiant magnetoresistance (GMR Giant Magnetoresistance) device, inparticular, relates to the integration of MTJ or GMR chips into astandard semiconductor package.

BACKGROUND ART

Magnetic sensors are widely used in modem systems to measure or detectphysical parameters including but not limited to magnetic fieldstrength, current, position, motion, orientation, and so forth. Thereare many different types of sensors in the prior art for measuringmagnetic field and other parameters. However, they all suffer fromvarious limitations well known in the art, for example, excessive size,inadequate, inadequate sensitivity and/or dynamic range, cost,reliability and other factors. Thus, there continues to be a need forimproved magnetic sensors, especially sensors that can be easilyintegrated with semiconductor devices and integrated circuits andmanufacturing methods thereof.

Magnetic tunnel junction (MTJ) sensors have the advantages of highsensitivity, small size, low cost, and low power consumption. AlthoughMTJ devices are compatible with standard semiconductor fabricationprocesses, methods for building high sensitivity devices with sufficientyield for low cost mass production have not been adequately developed.In particular, yield issues due to difficulty in MTJ process and backendpackaging process, and difficulty in matching the magnetoresistiveresponse of MTJ elements when combined to form bridge sensors haveproven difficult.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a process forpreparing a hi performance multi-chip, push-pull bridge linearmagnetoresistive sensor in a standard semiconductor package utilizingMTJ or GMR sensor chips and standard semiconductor fabricationprocesses. In order to achieve the above stated goals, one aspect of thepresent invention is to provide an single package bridge-type magneticfield sensor, comprising one or more pairs MTJ or GMR magnetoresistivesensor chips, wherein the sensor chips are fixed on a die attach area ofa standard semiconductor package, and each sensor chip includes a fixedresistance value reference resistor and a sensing resistor that changesin response to an external magnetic field. Both the reference resistorand the sensing resistor include a plurality of interconnected MTJ orGMR sensor elements in the form of an array.

Additionally, each of the reference resistor and the sensing resistor ismagnetically biased using a strip-shaped permanent magnet, sittingbetween the columns in the sensor arrays. The resistance value of thesensing resistor depends linearly on the external magnetic field. Thesensor chips include bond pad used to electrically connect the sensorchips to the die attach area and adjacent sensor chip using a pluralityof bonding lines in order to constitute a bridge sensor. The sensorchips and a leadframe are encapsulated to form a standard semiconductorpackage.

Another aspect of the present invention is to provide a single packagebridge-type magnetic field sensor, the sensor comprises one or morepairs of MTJ or GMR magnetoresistive sensor chips, wherein the sensorchips are fixed onto a die attach area of a standard semiconductorpackage; each sensor chip includes a fixed reference resistor and asensing resistor that responds to an external magnetic field; each ofthe reference and the sensing resistors comprises a plurality of MTJ orGMR magnetoresistive elements, wherein the MTJ or GMR magnetoresistiveelements are arranged in a matrix and interconnected to form a singlemagnetoresistive element;

the resistance of the sensing resistor is linearly proportional to theexternal magnetic field; the sensor chips include bond pads so that eachpin of the magnetoresistance elements can be connected to a bondingwire; the bonding wires are used to interconnect the sensor chips and toconnect the sensor chips to the die attach area in order to enable thefabrication of a bridge sensor. A leadframe and the sensor chips areencapsulated in plastic to form a standard semiconductor package.

Compared with the prior art, the present invention is advantageous: itdescribes a method for manufacturing a linear high-performancemagnetoresistive sensor that is easy to manufacture, low cost, andsuitable for mass production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—A schematic drawing of the magnetoresistive response of aspin-valve sensing element with the reference layer magnetizationpointing in the negative H direction.

FIG. 2—A schematic drawing of a half-bridge with a fixed referenceresistor and a sensing resistor.

FIG. 3—An embodiment of a half-bridge in a magnetoresistive sensor chipwhere both reference resistor and sensing resistor made of plural MTJelements arranged in row arrays and bar-shape permanent magnets are usedto bias the MTJ elements.

FIG. 4—An embodiment of a half-bridge in a magnetoresistive sensor chipwhere both reference resistor and sensing resistor made of plural MTJelements arranged in matrices.

FIG. 5—A drawing of half-bridge magnetoresistive sensor chip placedwithin a standard semiconductor package.

FIG. 6—A schematic drawing of a full-bridge sensor.

FIG. 7—A drawing of a full-bridge sensor with two half-bridgemagnetoresistive sensor chips placed within a standard semiconductorpackage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

These sensor elements are configured as spin valves, where one of themagnetic layers has a magnetization with an orientation that is fixed inorder to serve as a reference. This fixed layer can be a single magneticlayer or a synthetic antiferromagnetic structure composed of a pinnedferromagnetic layer exchange coupled to a reference ferromagnetic layer,wherein the pinned ferromagnetic layer is made magnetically insensitiveby exchange coupling it to an antiferromagnetic layer. The othermagnetic layer, the so called free layer, rotates in response to anapplied magnetic field. The resistance of the spin valve varies inproportion to the relative difference in the orientation of themagnetization of the free and pinned ferromagnetic layers. Because thefree layer rotates in response to the applied magnetic field, the sensoris sensitive to the applied magnetic field. In a MTJ element, the freelayer and fixed layer are separated by a tunnel barrier. Electricalcurrent flows through the tunnel barrier. In a GMR element, the freelayer and the pinned layer are separated by a non-magnetic metalliclayer. Electrical current can flow either in the plane of the multilayerthin film or perpendicular to the plane.

The general form of the magnetoresistive transfer curve of a GMR or MTJmagnetic sensor element suitable for linear magnetic field measurementis shown schematically in FIG. 1. The transfer curve depicted in thefigures saturates at low 1 and high 2, resistance values, R_(L) andR_(H), respectively. In the region between saturation, the transfercurve is linearly dependent on the applied magnetic field, H.

In non-ideal cases, the transfer curves is not symmetric about the H=0point in the plot. The saturation fields 4 and 5 are typically offset byan amount that is determined by the interlayer coupling between the freelayer and the pinned layer. A major contributor to the interlayercoupling, so called Neel coupling or “orange-peel” coupling, is relatedto roughness of the ferromagnetic films within the GMR and MTJstructures, and it is dependent on materials and manufacturingprocesses.

Between the saturation fields, 4 and 5, is the operation field regionwhere ideally the response of the MTJ or GMR resistance is linear.Sensitivity of the MTJ element, the slope 3 of the transfer curve inFIG. 1, depends upon the stiffness of the free layer in response to theapplied magnetic field. The slope 3 can be tuned by the shape of the MTJelement, to achieve the field sensitivity for specific designs andpurposes. Usually MTJ element is pattern into an elongated shape such asbut not limited to ellipse, rectangle, and diamond, orientedorthogonally with respect to the pinned layer. In some cases, the freelayer can be biased or stabilized by a permanent magnet in the directionperpendicular to the pinning layer. In some cases for high fieldsensitivity, magnetic field concentrators, or flux guides, can beintegrated in the magnetic field sensor to amplify the magnetic field onthe free layer of the MTJ elements.

FIG. 2 shows the schematic of a half-bridge configuration 10 with a biasvoltage 15 on a series of a reference resistor 13 with a fixedresistance and a sensing resistor 14 of which the resistance responds tothe applied magnetic field. The output voltage 12 is then the voltagedifference across the sensing resistor.

FIG. 3 shows a design of a half-bridge in a magnetoresistive chip 20.Both reference resistor 23 and sensing resistor 24 are composed of aplurality of MTJ elements, 231, and 241, respectively, which arearranged several columns. MTJ elements are connected in series to formthe reference resistor and sensing resistor. in between the MTJ elementcolumns there are bar-shaped permanent magnets 26 (PM) to bias the MTJfree layers in the direction perpendicular to the pinned layer.

In this case, the PM bars are oriented in the pinned layer magnetizationdirection. In chip fabrication, the PM's must be magnetized in thedirection perpendicular to the pinned layer in order to providestabilization field for the free layers. The PM's are not necessarilyfabricated in the same plane of the MTJs. However, they should be closeto provide sufficient bias field strength. Since the reference resistorshould not be sensitive to the applied magnetic field, the referenceelements 231 can be with different shape and/or different shape aspectratio from the sensing MTJ element 241 in order to obtain shapeanisotropy and magnetic stiffness against applied field. Alternatively,a magnetic shield 27 can be integrated in the chip to screen magneticfield/flux for the reference MTJ elements. In general, the magneticshield is a piece of soft magnet placed on top of the reference MTJelements, covering all the elements so that it shields the magneticfield from the elements and the fringe field of the shield at the edgeswill not affect the MTJ elements.

FIG. 4 shows a second design of a half-bridge in a magnetoresistive chip30. Both reference resistor 33 and sensing resistor 34 comprise aplurality of MTJ elements, 331, and 341, respectively, which arearranged in matrix configuration to achieve large area utilization. MTJelements are connected in series for both the reference resistor andsensing resistor. Since the reference resistor should not be sensitiveto the applied magnetic field, the reference MTJ elements 331 can bewith different shape and/or difference shape aspect ratio from thesensing MTJ element 341 to obtain shape anisotropy and magneticstiffness against applied field. Alternatively, a magnetic shield 37 canbe integrated in the chip to screen magnetic field/flux for thereference MTJ elements. In general, the shield is a piece of soft magnetplaced on top of the reference MTJ elements, covering all the elementsso that it shields the magnetic field from the elements and the fringefield of the shield at the edges will not affect the MTJ elements.

FIG. 5 is a drawing of a half-bridge magnetoresistive chip 43 placed andconnected in a standard semiconductor package. Wire bonding technique isused for the connection. The magnetoresistive sensor chips are wirebonded to each other and the leadframe. The half-bridge chip can be oneof the above embodiments in FIGS. 3 and 4. The field sensing direction46 is also shown with respect to the package orientation 47.

FIG. 6 shows the schematic of a full-bridge 50 that is essentially twohalf bridges, one includes reference resistor R_(ref1) 531 and senseresistor R_(s1) 541, and the other includes reference resistor R_(ref2)532, and sense resistor R_(s2) 542, connected in parallel betweenV_(bias) 55 and GND 51. The output is the voltage difference between V₊and V⁻.

FIG. 7 is a drawing of a full-bridge sensor composed of two electricallyinterconnected magnetoresistive chips 631 and 632 placed in a standardsemiconductor package. Wire bonding technique is used to make theelectrical connections. The magnetoresistive sensor chips are wirebonded to each other and a die attach area, The type of the twomagnetoresistive chips can be one of the above embodiments in FIGS. 3and 4. The field sensing direction 68 is also shown with respect to thepackage orientation 67. In this full-bridge sensor embodiment, the twomagnetoresistive chips are oriented opposite to each other so that theresponse of the sensing resistors to applied magnetic field is oppositein polarity. Since the resistance of the reference resistors and of thesensing resistors in zero field should match each other well, all theMTJ elements will be completed in the same fabrication process.Furthermore, the shape and/or shape aspect ratio of the MTJ elements forreference resistor and for sensing resistor can only be adjusted underthe constraint for resistance matching.

A push-pull full bridge sensor can provide higher sensitivity and largeroutput voltage than a conventional full bridge sensor. Instead of havingtwo reference resistors with fixed resistance, the push-pull full bridgeis configured in the way that all the four resistors respond to theapplied magnetic field.

It will be apparent to those skilled in the art that variousmodifications can be made to the proposed invention without departingfrom the scope or spirit of the invention. Further, it is intended thatthe present invention cover modifications and variations of the presentinvention provided that such modifications and variations come withinthe scope of the appended claims and their equivalence.

What is claimed is:
 1. A single-package bridge-type magnetic fieldsensor, comprising: one or more pairs of MTJ or GMR magnetoresistivesensor chips, wherein the sensor chips are adhered to a die attach areaof a standard semiconductor package; wherein each of the sensor chipsincludes a reference resistor with a fixed resistance and a sensingresistor with a resistance varying in response to a magnetic field;wherein each of the reference resistor and the sensing resistor includesa plurality of MTJ or GMR magnetoresistive sensor elements electricallyinterconnected as a single magnetoresistive element in a matrix, thematrix including adjacent rows of sensor elements and adjacent columnsof sensor elements where adjacent magnetoresistive elements in eachcolumn are directly connected in series; and wherein the sensingresistor has a transfer curve that is linearly proportional to anapplied magnetic field in an operating magnetic field range.
 2. Thesingle-package bridge-type magnetic field sensor as in claim 1, whereinthe sensor is a half-bridge sensor comprising one sensor chip.
 3. Thesingle-package bridge-type magnetic field sensor as in claim 1, whereinthe sensor is a full-bridge sensor comprising a pair of sensor chips,wherein one of the pair of sensor chips is rotated 180 degrees withrespect to the other.
 4. The single-package bridge-type magnetic fieldsensor as in claim 1, the magnetoresistive elements are patterned in astrip-like shape including but not limited to an elliptical, arectangular, or a diamond shape.
 5. The single-package bridge-typemagnetic field sensor as in claim 1, wherein the magnetoresistiveelements of the reference resistor are patterned in a different shapeaspect ratio from that of the magnetoresistive elements of the sensingresistor.
 6. The single-package bridge-type magnetic field sensor as inclaim 1, wherein the reference resistor is screened from an appliedmagnetic field by one or more magnetic shields.
 7. The single-packagebridge-type magnetic field sensor as in claim 3, wherein the sensorchips are tested and sorted before assembly in order to better matchtheir transfer curve characteristics.
 8. The single-package bridge-typemagnetic field sensor as in claim 1, wherein the sensor comprising aplurality of bond pads of the sensor chips are designed such that morethan one wire bond may be attached to each side of the magnetoresistiveelements.
 9. The single-package bridge-type magnetic field sensor as inclaim 1, wherein the magnetoresistive sensor chips are wire bonded toeach other and a leadframe in order to produce a bridge sensor.
 10. Thesingle-package bridge-type magnetic field sensor as in claim 9, whereinthe leadframe and sensor chips are encapsulated in plastic to form astandard semiconductor package.